Update apiInvoker.mustache and sample file for akka-scala client for issue #7258 fix (#7259)

* Update apiInvoker.mustache

In scala-akka-client code that is getting generated, addAuthentication method is called after setting headers using header parameter in the below mentioned line
addAuthentication(request.credentials)(
      httpRequest.withHeaders(headers(request.headerParams))
    )

However, in addAuthentication method, we are using withHeaders method that overwrites the headers set using header parameters. So, I am proposing to
change the addAuthentication method be replacing withHeaders() method to addHeader() to add authentication header to the list of already existing headers.

* Update ApiInvoker.scala

Added changes to sample for the client generator code change

Author: ParakhMittal

modules/openapi-generator/src/main/resources/scala-akka-client/apiInvoker.mustache

@@ -103,11 +103,11 @@ class ApiInvoker(formats: Formats)(implicit system: ActorSystem) extends CustomC
     request: HttpRequest =>
       credentialsSeq.foldLeft(request) {
         case (req, BasicCredentials(login, password)) =>
-          req.withHeaders(Authorization(BasicHttpCredentials(login, password)))
+          req.addHeader(Authorization(BasicHttpCredentials(login, password)))
         case (req, ApiKeyCredentials(keyValue, keyName, ApiKeyLocations.HEADER)) =>
-          req.withHeaders(RawHeader(keyName, keyValue.value))
+          req.addHeader(RawHeader(keyName, keyValue.value))
         case (req, BearerToken(token)) =>
-            req.withHeaders(RawHeader("Authorization", s"Bearer $token"))
+            req.addHeader(RawHeader("Authorization", s"Bearer $token"))
         case (req, _) => req
       }
   }

samples/client/petstore/scala-akka/src/main/scala/org/openapitools/client/core/ApiInvoker.scala

@@ -113,11 +113,11 @@ class ApiInvoker(formats: Formats)(implicit system: ActorSystem) extends CustomC
     request: HttpRequest =>
       credentialsSeq.foldLeft(request) {
         case (req, BasicCredentials(login, password)) =>
-          req.withHeaders(Authorization(BasicHttpCredentials(login, password)))
+          req.addHeader(Authorization(BasicHttpCredentials(login, password)))
         case (req, ApiKeyCredentials(keyValue, keyName, ApiKeyLocations.HEADER)) =>
-          req.withHeaders(RawHeader(keyName, keyValue.value))
+          req.addHeader(RawHeader(keyName, keyValue.value))
         case (req, BearerToken(token)) =>
-            req.withHeaders(RawHeader("Authorization", s"Bearer $token"))
+            req.addHeader(RawHeader("Authorization", s"Bearer $token"))
         case (req, _) => req
       }
   }